1) Field of the Invention
The present invention relates to a direct conversion receiver for use in mobile radio equipment or the like, a mobile radio equipment using this direct conversion receiver, and a method of receiving an RF signal.
2) Description of the Related Art
FIG. 5 is a block diagram showing a circuit arrangement of a conventional typical direct conversion receiver. In this illustration, a quadrature demodulator 1 is made to receive, as one input, an RF (Radio Frequency) signal obtained by the quadrature modulation of carriers in phase and amplitude and to receive a local signal Lo as another input for outputting signals i, ix (i and ix denote a differential signal) and q, qx (q and qx signify a differential signal) pertaining to a baseband, which have quadrature relation to each other. A receive baseband unit 2 is connected to output terminals of the quadrature demodulator 1 for the baseband signals i and ix, and a receive baseband unit 3 is connected to output terminals thereof for the baseband signals q and qx. These receive baseband units 2 and 3 have the same arrangement, and a detailed description will be given of only the arrangement of the receive baseband unit 2 for the purpose of the simplicity of the illustration and explanation. The receive baseband unit 3 is illustrated as a single block.
The receive baseband unit 2 is made up of a low pass filter (LPF) 21 for deriving low-frequency components of the baseband signals i and ix, a gain control amplifier 22 for amplifying an output signal of the low-pass filter 21, an amplifier 25 for amplifying an output signal of the gain control amplifier 22 at a constant gain to output baseband signals I and Ix (I and Ix represent a differential signal), and an offset compensating circuit 20 for removing a DC offset occurring in the receive baseband unit 2. This offset compensating circuit 20 is composed of a low pass filter (LPF) 24 for receiving the baseband signals I and Ix outputted from the amplifier 25 to derive low-frequency components therefrom, and an offset compensating section 23 for receiving signals outputted from the low-pass filter 24, that is, signals corresponding to offset voltages, to convert them into voltages or currents for feedbacking the voltages or the currents to an input of the amplifier 25. Thus, the baseband signals I and Ix, subjected to the removal of the DC offset, are fed to a control unit 4.
In addition, DC offset removed baseband signals Q and Qx (Q and Qx depict a differential signal) are supplied from the receive baseband unit 3, having the same arrangement as that of the receive baseband unit 2, to the same control unit 4.
The control section 4 calculates a received signal level on the basis of the given baseband signals I, Ix and Q, Qx to apply a gain control signal Vgc to the gain control amplifier 22 forming a component of each of the receive base band units 2 and 3 and further to a clock signal (which will be referred to hereinafter as a “CLOCK”), a data signal (referred to hereinafter as a “DATA”) and a strobe signal (referred to hereinafter as a “STROBE”). A frequency synthesizer 6 produces a local signal Lo from a reference signal of a reference signal source 5 on the basis of data set by the CLOCK, the DATA and the STROBE and supplies the local signal Lo to the quadrature demodulator 1. Since this frequency synthesizer 6 requires high stability and high signal purity, a PLL (Phase Locked Loop) is constructed with a voltage control oscillator 61 and a frequency control section 62.
Secondly, a description will be given hereinbelow of an operation of the conventional direct conversion receiver shown in FIG. 5.
In a case in which a direct conversion receiver is employed for a mobile radio equipment based on a W-CDMA (Wideband-Code Division Multiple Access) system, since there is a need to receive a transmitted signal linearly irrespective of the strength of the received signal, a receive baseband unit is required to have performances, such as a cascade gain of several tens dB and a gain control range of several tens dB.
Now, when an RF signal obtained by the quadrature modulation of the phases and amplitudes of carriers and an output signal Lo of a frequency synthesizer are inputted to the quadrature demodulator 1, the quadrature demodulator 1 outputs baseband signals i, ix and q, qx. Of these signals, the signals i and ix are inputted to the receive baseband unit 2 while the signals q and qx are put in the receive baseband unit 3. A description will be given of only the receive baseband unit 2, for that the receive baseband units 2 and 3 have the same arrangement.
At this time, the signals i and ix undergo the band restriction in the low pass filter 21 and are amplified in the gain control amplifier 22 and further amplified at a constant gain in the amplifier 25 to be inputted as signals I and Ix to the control unit 4. The control unit 4 calculates a received signal level on the basis of the signals I and Ix to output a gain control signal Vgc for controlling the gain of the gain control amplifier 22 so that the values of the signals I and Ix become constant.
Also in the receive baseband unit 3, as well as the receive baseband unit 2, the signals q and qx are amplified so that the constant-level signals Q and Qx are inputted to the control unit 4.
At this time, the control unit 4 sets and outputs a CLOCK, a DATA and a STROBE, and the frequency synthesizer 6 produces a local signal Lo from an output signal of the reference signal source 5 on the basis of these set data, and sends it to the quadrature demodulator 1.
When the quadrature demodulator 1 mixes the local signal Lo into an RF signal, offset voltages occur between the signals i and ix and between the signals q and qx. Even if these offset voltages are several mV, each of the receive baseband units 2 and 3 has a cascade gain of several tens dB, which can make difficult the normal reception because of the occurrence of circuit saturation in the control unit 4.
The offset compensating circuit 20 resides therein for the purpose of avoiding this problem. This offset compensating circuit 20 inputs the output signals I and Ix of the amplifier 25 to the low pass filter 24 to extract a DC voltage through the low pass filter 24. In addition, the offset compensating section 23 feedbacks voltages or currents corresponding to the inputted difference voltages (offset voltages) to an input section of the amplifier 25. This feedback loop operates to eliminate the DC voltage offsets between the signals I and Ix.
When the low pass filter 24 is included in this feedback loop, the closed loop thereof provides a high pass filter characteristic, with the cutoff frequency of the high pass filter lowering with an increase in the time constant of the low pass filter 24. Accordingly, in the output signal of the receive baseband unit 2, a signal component in the vicinity of the DC is cut off due to the effects of the high pass filter. That is, the receive error rate becomes high. The time constant of the low pass filter is set in consideration of the high pass filter characteristic.
FIG. 6 is a time chart corresponding to the above-described operations. In the illustration, CLOCK, DATA and STROBE signify a clock signal, a data signal and a strobe signal, respectively, with desired frequency data being set in the frequency synthesizer 6 on the basis of these signals, and Lo depicts a local signal outputted from the frequency synthesizer 6 and represents a state of variation in frequency of the output signal of the voltage control oscillator 61. The frequency of this local signal Lo stably becomes a desired frequency at THE time elapsed to some extent with respect to the STROBE, that is, at the time instructed by a lock signal (LOCK). Moreover, Vgc denotes a gain control signal outputted from the control unit 4 which controls the gain of the gain control amplifier 22 in accordance with a change of its level so that the output signals of the receive baseband units 2 and 3 reach predetermined levels, respectively. In this arrangement, assuming that the gain of the gain control amplifier 22 is increased in a state where the gain control signal Vgc is at a high voltage level, FIG. 6 shows a state in which the gain is stepwise increased when the received level is low.
In addition, in FIG. 6, I signifies a variation in DC level of a signal I with the passage of time. Properly, although an AC signal should be illustrated additionally therein, only the DC level is boldly shown for the purpose of the simplicity of explanation. A signal Ix has a waveform obtained by inverting the signal I upside down, and signals Q and Qx have waveforms similar to the signals I and Ix, respectively. When the gain control signal Vgc varies, the gain of the gain control amplifier 22 varies, so the DC level of the signal I varies instantaneously. Following this, the DC level of the signal I develops into one value at a response speed depending upon the time constant of the low pass filter 24.
In the above-described direct conversion receiver, when the signals I, Ix, Q and Qx vary in DC at the gain control of the gain control amplifier 22, a problem arises in that demodulation accuracy deteriorates. For this reason, the calculation of the received signal level is required to be done after the settlement of the DC variation, and the response time (the time taken until a gain control signal Vgc is set on the basis of the signal level calculated from the signals I, Ix, Q and Qx so that the variation in level of the signals I, Ix, Q and Qx comes to an end) of the automatic gain control (AGC) is limited by the response time of the offset compensating circuit 20. Accordingly, immediately after the frequency switching operation, an extremely long time is needed until the signals I, Ix, Q and Qx reach a predetermined AC level with the gain control being implemented by the automatic gain control.
In addition, in the case of the gain control for each receive slot during calls or the like, difficulty is encountered in avoiding the deterioration of the demodulation accuracy due to the DC variation.
Still additionally, since the increase in number of control signals to be outputted from the control unit 4 can be inhibited depending upon mobile radio equipment using this direct conversion receiver, there is a need to simplify the control section and the interface as much as possible.
Moreover, in a case in which this direct conversion receiver is employed for a mobile radio equipment such as a portable telephone, since the settlement of the automatic gain control during a queue operation takes time, there occurs a problem of prolonging the operation time of the receiver during the wait operation and shortening the wait time.
Still moreover, the call quality degrades due to the DC variation at the gain control in a call.